Conventionally, a compressor provided with a speed-up mechanism is known as disclosed in the following Patent Document 1. Using the speed-up mechanism arranged between the driving shaft of an electric motor and the main shaft of a compression portion, the compressor is capable of driving the compression portion at a higher rotational speed than the electric motor while lowering the rotational speed of the electric motor. The compression portion includes a diffuser extending in the radial directions which reduces the flow velocity of a working fluid accelerated and pressurized by an impeller of the compression portion, and thereby, the compressor discharges the working fluid at a predetermined velocity reduced by the diffuser.
The compressor disclosed in the following Patent Document 1 cannot be miniaturized beyond a certain limit. Specifically, the speed-up mechanism provided for the compressor requires that a first gear provided in the rotating shaft of the electric motor should have a larger diameter to thereby rotate the main shaft of the compression portion at a higher speed than the driving shaft of the electric motor and also requires that the electric motor should be arranged offset against the compression portion to thereby engage the first gear and a second gear provided in the main shaft of the compression portion. This enlarges the width of the compression portion in the diametrical directions and hence sets limits to miniaturization of the compressor or particularly an axial flow compressor. Besides, the diffuser provided in the compression portion extends in the diametrical directions with respect to the impeller, thereby enlarging the width of the compression portion in the diametrical directions.